The ‘Copy Material Instance Parameters’ node is marked as ‘Very slow implementation, avoid using at runtime’ - but I need to copy material parameters from one dynamic instance to other one in-game. So will it be more performant if I would copy all the parameters (around ~20 parameters) by hand, one by one?
Depends on your needs, and target platform. But that goes with almost any decision within game development. If your overall performance is not impacted by this node, just use it. If you realize at some point this is not running fast enough (e.g. maybe you need to call it every frame), look if you can store the parameters yourself and make a function to copy them.
As a start I would call a function which just calls this node, If you need to replace it at any point you can just rework the function instead of finding all the places where you called the node.
I target desktop and just did some tests, looks like this node isn’t that bad as it’s described. I’ll look into it again if I’ll have some performance problems. For now I’ll just use it.
Yeah, storing it in a function is a good programming practice in general 
Thanks!